This paper describes extensive computer-based analytical studies on the details of unsteady flow behavior around oscillating airfoils in turbomachinery. To consider the time-dependent motions of airfoil, a complete Navier-Stokes solver incorporating a moving mesh was applied, and the drag and lift coefficients for the cases of stationary airfoils and airfoils subjected to forced oscillation were examined. In the present study, a comparison was made between the two-and three-dimensional analyses for NACA0012 airfoils, and the separation vortex structure was examined in detail. From the numerical results, it was found that the separation vortex consisted of large-scale rolls with axes in the span direction, and rib substructures with axes in the stream direction. The three-dimensional analysis could simulate these rolls and ribs, but the two-dimensional analysis was inadequate to realize this vortex structure. This is the main difference between the two-and three-dimensional analyses. In addition, the formation of ribs was found to be affected by the forced oscillation, and the transformation or rolls increased and the vortex structure became fine as the oscillation frequency increased.